Instructions
Complete the main game and save your file. This will unlock "The Mercenaries" game mode.
Obtain a five-star rating on all four maps of "The Mercenaries" using all all five characters. That is, you must get a five-star rating on every map using Wesker, Hunk, Ada, Leon and Krauser.
Load a new game using your cleared game save file. The hand cannon is now available from the merchant for free.